DESCRIPTION OF RELATED ART
Some modern ski poles utilize high performance composite materials to increase the strength and stiffness of the pole while reducing its weight relative to traditional aluminum poles. High performance composite materials have a lower density, higher specific strength and stiffness, and better damping qualities than traditional metals. Examples of higher performance reinforcing fibers include carbon or graphite fiber, Kevlar fiber, and boron fiber. When the continuous fiber type variety of these fibers are used in composites in high fiber volumes, generally greater than 50%, the resultant composite material will generally have mechanical and strength properties significantly better than metals for a given weight.
These properties are beneficial in the design and manufacture of high performance ski poles. High performance composite ski poles have been in use for cross country ski poles for over 15 years, but these are different from the method and the products disclosed here. The use of high performance composite materials in alpine or downhill ski poles has increased dramatically over the last five years. The increase in impact resistance in downhill poles which has been obtained by development of better pole designs has been the primary factor allowing more widespread use of composites in downhill ski poles, as has the consumer's increased demand for higher performance products. The impact conditions in the downhill ski environment are significantly greater than those experienced by a high performance cross country ski pole.
The vast majority of ski poles are of a circular cross section. Many of these are also longitudinally tapered, thereby having circular cross sections of decreasing size from the handle to the tip of the pole. Oval, elliptical, or aerodynamic ski poles have been developed previously, but have never been commercially successful. This has been caused by lack of a suitable manufacturing process capable of producing well designed non-circular cross section or bent centerline ski poles at commercially acceptable costs.
The invention comprises a manufacturing method and pole construction, which is particularly suitable for poles having varying non-circular cross sections and/or having curved centerlines. The invention provides a commercially viable way of manufacturing these types of pole designs, where none has existed previously. The manufacturing process utilizes a fundamentally different manufacturing approach than any known currently commercially utilized ski pole manufacturing process, and is also significantly different from, and unanticipated by any known process described in the technical or patent literature.
There appear to be no internal pressure bladder ski poles in the prior art. There are several determinates of a manufacturing process, the two key defining characteristics of a manufacturing process are how the fiber/resin material is positioned, and how pressure is applied. The internal pressure bladder process substantially changes the nature of both of these key aspects of the process relative to prior an.
The internal pressure bladder process initially positions the material on a very flexible substrate (probably the most flexible substrate possible) and the position of the material changes (it moves) significantly during the cure. The movement of the material inside the mold is far greater than any existing processes. Existing processes may move the material tens of thousands of an inch at most, whereas the process of the present invention moves the material at least a tenth of an inch, i.e. a factor of 100 increase.
The pressure is clearly applied completely by the pressure bladder in our process. The pressure bladder can be considered loosely analogous to, i.e. the interior tool, customarily a fixed dimension mandrel, in most prior art processes. The invention's application of pressure on the interior of the part can be controlled completely independent of any other process variable. The vast majority of composite tube making processes, including all known commercial processes making parts in any significant volume, apply the pressure to the outside of the part. Certain relatively short tapered tubes in non-analogous fields such as for vehicle frames use internal pressure bladders, but these are subject to different loads and forces than poles, as the are bonded at both ends, and the ends bear the greatest stress.
If a rigid interior mandrel and exterior tool are used, the pressure is applied perhaps equally by both tool portions, and is a function of some sort of relative mold motion (i.e. mold halves are closed), or the injection of liquid resin. Applying pressure through relative mold motion, i.e. closing the mold, provides generally external pressure and is significantly different from the process described in the present invention.
This is also a clumsy approach since fibrous material tends to flow out of the mold as it is closed, if a split female tool is used. If split female tooling is not used, there are very strong limitations on the geometric shape of the part.
Another alternative, believed to be used only in non-analogous arts, which provide some internal pressure is an expanding foam core. In this process, the skin is placed in a mold, and thermally (by heating or chemically) expanding foam placed inside the skin. When this is used, then the pressure application is directly related to the temperature of the tool, and not independently controllable. This severely limits process control, and maximum pressure levels. Further, the foam core is not generally removable, which is a huge disadvantage because of the added weight of the foam.
The composite ski pole industry is very heavily oriented to the use of hard internal mandrels, with pressure applied to the outside of the part during cure. These are believed to all use hard interior mandrels where at least the fiber is distributed precisely on the mandrel in a position very close to, if not exactly, where it will be in the finished part. The process of the present invention does not use a hard interior mandrel.
The other commonly used manufacturing method used for pole manufacture is pultrusion. Pultrusion is a fundamentally different process from that of the present invention. Several patents include manufacturing via pultrusion, although their primary teaching is not on pultrusion.
Several patents disclose a hard female tool in addition to the hard interior tool. These also use a hard female tool without split lines. The use of a hard interior mandrel, still make these approaches quite different than that of the present invention. The pressure application methods are also quite different. In these, the fiber or fiber/resin is positioned close to its final position, since the rigid tool doesn't change shape or expand significantly. There is teaching from one or more of these references, that split molds are undesirable, which is believed to teach away from this invention. Split line molds can create undesirable effects if rigid interior tooling is used, and the pole preform/interior tooling assembly is not smaller diametrically than the final pole dimensions. Because the present inventions utilizes a flexible interior tool with a pole preforms which is smaller than the final dimensions these problems are alleviated.
Some prior art patents teach the use of matched female tools and semirigid interior tooling, but these use foam cores. These patents use preforms formed initially very close their final shape, and do not use internal fluid pressurization. One uses an interior mandrel to supply the compaction pressure, but as discussed above, the limitations on the production of foam cored poles are severe. Since compaction pressure is low, composite strength to weight is low. The poles are heavier for a given strength. This and the fact that the foam must remain, makes for a much less desirable ski pole.
The fiber material in the present invention moves significantly during cure, and the original positioning of the fiber/resin skin, preferably a prepreg, on the bladder, prior to insertion in the tool, need not be very precise. The fact that one can relatively roughly apply the prepreg material (in a positional sense) on the bladder, in a shape significantly smaller than the finished part, and then move the material to its final shape during cure, is believed to be a considerable advantage in manufacturing efficiency, as well as being related to providing a stronger lighter pole because of the reduction of voids between prepreg laminations or layers.
Using this method reduces the labor and precision required to arrange the prepreg when fabricating the pole. This method allows the preform bladder assembly to be prepared out of the tool in a separate step and stored for a later cure. This method also allows the loading of hot molds with the bladder preform assembly which means the mold does not have to be cooled between cycles, significantly increasing production rate compared to forming the material on the hard tooling, either internal mandrel or female tool.
The method of the invention further eliminates, or substantially reduces, fiber flash during cure. Little or none of the fibrous part of the part is caught between the mold halves, which greatly reduces finishing costs and increases part structural integrity.
The method of accomplishing the step of supplying heat or an elevated temperature for curing is sometimes regarded as important, and the method here enables greater control over that step as well. The avoidance of mold cooling, the precision of the fit of the mold halves or portions, and the preparation of the bladder and prepreg insert all complement the control of the heating or other curing step.
Other external pressure methods include those of winding bundles of filaments around a solid mandrel, or braiding fibers around a solid mandrel. These have no way of applying internal pressure, and pressure, if applied at all, may act from the limited tension in the winding process, or from a subsequent step such as covering the tube with heat shrinkable plastic tape, cellophane or high-temperature thermoplastic, and shrinking the tubing to supply external pressure. A somewhat analogous process used is referred to as table rolling, where the prepreg is rolled around a mandrel, and pressure applied externally between the mandrel and a surface such as a table.
This is the most common method for manufacturing composite ski poles. When mechanized this process uses a "rolling table" machine to wrap prepreg material around a rigid mandrel. Table rolled composite ski poles use a plastic tape, cellophane or high-temperature thermoplastic, wrapped tightly around the laminate to compact the laminate and apply pressure to the laminate during cure. In this case the compaction pressure is applied with the circumferential tension in the tape. This tape is commonly called "shrink tape", however it probably does not actually shrink during cure, the initial winding tension provides the compaction pressure. The compaction pressure provided by shrink tape is on the order of 10 psi. to possibly 30 psi.
One patent describes generally straight, oval ski poles which have different bending properties in forward and lateral directions.
The composite materials used are generally heat curable synthetic resins reinforced with structural fibers. More specifically, the composite material used in construction of the poles contains a plurality of organic fiber layers, the fibers composing each layer being oriented at a specific angle depending on the design requirement, and all of these sheets being impregnated with a resin which hardens to form the finished component. In the preferred embodiment the resin is preimpregnated into the fiber beds prior forming of the poles, hence the commonly used term for this material form is "prepreg" composite. Prepreg composite is commonly available in standard form with a variety of different resins and reinforcing fibers from several different manufacturers. It is not necessary to use the prepreg form of the composite material to utilize this invention. For example, it is possible to manually add the resin to dry fiber material immediate prior to forming the pole. Also, it is relatively simple to envision that the resin can be injected into a closed mold into which a dry fiber pole preform/bladder assembly has been loaded.